Onions

Overview

Onions – bulb                                         Rotation group  -     Roots

Traditional bulbing onions can be grown from seed or small bulbs called sets. There are advantages and disadvantages with both methods. Growing from seed always carries the risk of seed transmitted diseases such as onion white rot contaminating the onion bed but it does have the extra advantage of offering allowing you to choose from a wider selection of cultivars. If you want to grow the large exhibition size onions you must always buy the seeds from specialist growers that has been bred and selected over many years. Even though the bulbs are large they are still perfectly edible. Most people are surprised to discover just how mild tasting they are but they are hopeless for stringing and putting into store because they will not keep for more  than a couple of months. Whether growing from seed or sets always use the largest bulbs up first and select the smaller bulbs for storing.

 

The most popular method used to grow onions is from sets. These are small immature bulbs that are started into growth before they are stopped from developing any further when they are about ½ in/12.5 mm in diameter. To prevent the possible transmission of any onion diseases they should  have been  heated treated to kill off diseases. Regretfully not all onion sets offered for sale are heat treated. Always check to find out and if they are not leave them alone and buy your sets from another source where they have been heat treated.

 

Overwintering onion sets are planted out during late September/October. They must not be confused with the spring planting cultivars. The cultivation methods for them are exactly are the same.

 

Varieties to choose (sold by Kings Seeds): -

How to grow

Seed sowing/planting

Onion seed can be sown as early as January if a heated greenhouse is available. They require a minimum temperature of 60F/16C to germinate. They can then be grown on at a lower temperature of 50F/10C by day and 45F/7C at night.

Sow the seed in pots or trays filled with fresh seed compost lightly covering the seed with a layer of fine seed compost. Place the pots or tray in a heated propagator set a 60F/16C until the seedlings begin to show through the compost; at this stage they must be removed from the propagator and grown on at the lower temperature to prevent them from becoming soft and leggy. Just as the seedlings begin to unfold, known as the ‘shepherds crook’ stage they are ready for pricking out either singly into 3 ½ “/9cms pots or spaced out 3”75mms apart in deep seed trays filled with fresh potting compost.

The onions seedlings can remain in the pots or trays until planting out time during March/April. If the weather conditions delay the transplanting they will need to be watered with a liquid general fertiliser such as sea weed to keep them in good condition until they can be transplanted. It is important to fully harden the plants off before planting them out to avoid any check to their growth.

 

Onion seed can be sown in the open ground during March and the seedlings grown on to be transplanted singly in rows towards the end of April.

The seed is sown on a prepared seed bed in ½ “/12 mm deep drills. Sow the seed thinly along the drill to minimise having to thin out the seedlings. When the seedlings are 6”/15cms tall they are ready for transplanting. Choose a cool, cloudy spell of weather to carry out the transplanting to avoid putting the young plants under stress.

Growing onions from sets

 

Summer growing onion sets are available from February until the end of March. They should be planted out by the end of March but if this isn’t possible then plant them out as early in April as possible.  Always remove them from any packaging, spread them out in a seed tray and keep somewhere light, cool and airy. If they get too warm they will break into growth before the conditions outdoors are suitable for planting them out.  Discard any damaged sets or those  that are more than ½ “/12mm in diameter and any very small bulbs also those that are already producing leaf shoots. A simpler and safer method of storing the onions is to grow then on in deep seed trays filled with fresh potting compost. This will buy them time until the weather and soil conditions are suitable for transplanting during March/April. They can safely be grown on in an unheated greenhouse but will need some protection on extremly frosty nights. Although they will have been grown in tough conditions they will still need to go through the hardening off process before planting out.

Transplanting/Planting out

The onion plants grown from seed will be around 6”/15cms tall and ready for planting out by March. If grown in a pot they will suffer very little root disturbance but the plants grown in trays will be have to be handled more carefully. Try to keep the root damage to a minimum and transplant them with the base of the plantjust below the surface of the soil. If they are planted too deeply they will form thick necks making the onion useless for storing.

 

Onion sets grown in boxes are treated in exactly the same way as the onions grown from seed except the top of the set is just at the surface of the soil. If the sets have been kept loose in a tray they should have the loose, strawy top cut off to prevent birds pulling the sets out of the soil before they have had a chance to produce roots.  The onion sets must not be pushed into the soil because it will damage the basal plate where the roots are formed. If the base is damaged it increases the risk of infection.

Both the transplants and the sets are planted using a trowel. Stretch a garden line out across the plot and plant along the line. In both cases the distance between the rows should be 12”/30cms with a minimum 6”/15cm to a maximum 12”/30cms between the plants/sets in the row. The larger the spacing between the plants the larger the onion bulb will be. Water the transplants with liquid seaweed mixture to settle them in and to avoid suffering from transplant shock.

Aftercare

Hoe between the plants regularly to control weeds and to create a dust mulch that will conserve soil moisture. If the soil isn’t in a suitable condition to hoe the bed must be hand weeded. Onions do not like competition from other plants for light, air, water and nutrients. Irrigate weekly and if conditions are hot or dry water or spray the plants with a liquid sea weed mixture. Take care when hoeing or working between the plants not to damage the bulb and foliage.

How to Harvest

Wait until the tops of the onions begin to fall over naturally don’t be tempted to bend them over this will only damage the neck of the bulb and cause a soft rot to set in. As the tops fall over lay the neatly between the rows a bit like ‘sardines’. The next step is to carefully push a garden fork under each onion bulb and gently lever it upwards to snap the roots. This will stop the bulb growing any further and tell it to start the ripening process.  After a week or two the entire onion plant is ready to be dug up. DON’T pull the onion out of the soil. If it is warm and the soil is dry lay the plants on the surface of the soil with the base of the onion facing south to dry off. If the soil is wet lay the plants in a greenhouse or frame to protect them. The leaves don’t have to be crisp at this stage they can be allowed to dry off over the coming weeks. Once the leaves have dried out completely they can be pulled off the plants. Remove the loose dry skins but don’ start peeling the tight layers of skin off the bulbs. The traditional way to store onions is to tie them to a string and hang them up. It is far more practical to store them in net sacks or trays.

 

Store the onions somewhere that is cool, frost free and light to keep them dormant over the winter months. If onions are stored in warmth they will break their dormancy and start to grow new leaves.

Issues

Onion fly. This is most active during May and June although attacks later in the summer are not uncommon. Take care not to damage the plants or leave any onion plant remains in the vicinity.

 

Basal White Rot Always worse in warm dry summers. Irrigate regularly to help prevent an attack. If there is an attack of basal white rot damaged plants must be dug up and destroyed sadly the soil will remain infected for up to 10 years making any further onion growing impossible. It is best to use a rotation system and create a fresh onion bed every year.

 

Powdery white mildew This is always worse in cool and wet summers.